Nowadays mobile communication networks are widely used. These communication networks comprise a plurality of network cells each having at least one base station used to receive and transmit signals from user equipment, e.g. mobile phones or PDAs. A plurality of different environments or systems are known, e.g. GERAN, UTRAN, LTE, E-UTRAN, WCDMA, or WLAN. For ensuring a good performance and in particular a secure data transmission it has to be ensured that all data, data signals, or data packets are received at the intended recipient and at possible relay stations, like base stations of the mobile communication network.
In order to increase the transmission quality so called Hybrid Automatic Repeat Request (HARQ) error correction methods can be employed. When using HARQ the transmitting side consecutively transmits transmission time intervals (TTI) belonging to the HARQ processes. The receive side attempts to decode each process and sends back an acknowledge (ACK) or a non acknowledge (NACK) message or signal, upon which the transmit side either transmits new data in the process and retransmits the data transmitted in the previous cycle of the process, respectively. The receiver soft combines multiple receptions of the same data protocol data unit (PDU).
In particular, one problem limiting the performance of the data transmission known in the prior art is inter cell interference. In order to reduce the inter cell interference some form of cooperative antenna (COOPA) systems are proposed. From theory significant performance gains with respect to capacity and coverage are known for full cooperating cellular radio systems compared to conventional ones. Due to these large gains, which cannot be achieved with other technologies, as it is known from theory that COOPA systems provide an upper bound for interference limited cellular radio systems. At the same time it is clear that full cooperation is not practical due to required channel state information (CSI) to a huge number of base stations (BS) and accordingly large feedback overhead, especially in case of FDD systems. Furthermore, in case of time division duplex (TDD) systems channel reciprocity might relieve feedback overhead a little bit, but for wide area frequency division duplex (FDD) systems this is an important issue. A further topic is the large amount of data on the backbone network, which might generate a lot of costs in terms of CAPEX and OPEX.
Different types of cooperative antenna systems (COOPA) have been proposed in the meantime, but in the following the basic cell of any cooperation area (CA), consisting of two cooperating Node Bs (NBs) and two user equipments (UEs) will be investigated. In case of cooperative transmission generally a central unit (CU) performs common signal precoding, which is basically a matrix multiplication of all data signals with a precoding matrix W. In case of zero forcing (ZF) W is the pseudo inverse H+ of the overall channel matrix H. The simplest form of a CA may be formed by a codebook based precoding. In this case the precoding matrix W is selected from a codebook based on the different preferred matrix indices (PMI) feedbacks PMI1 and PMI2 from the UEs UE1 and UE2.
Generally the radio channel matrix H will be time variant, which leads to a high sensitivity to feedback delay of PMI values. Unfortunately due to processing time at the transmitter as well the receiver and the need to wait for transmission opportunities for the PMI values there will be a delay of several subframes between the time of estimation of the radio channel to the time when the precoding with the precoding matrix W is applied to the user data. In the meantime the radio channel H will have changed so that the precoding will go wrong.
Different proposals exist to reduce overall feedback for MIMO as well as cooperative transmission schemes like subspace schemes, where the real radio channel dimension, which is most often much smaller than the one of the actual radio channel of the transmission is determined and only updates in the subspace dimensions is being sent.
Another direction is to use tracking algorithms, based on the assumption that radio channels will vary smoothly so that the difference between one measurement to the next is small and can be fed back by smaller number of bits.
However, in high performance solutions with significant precoding gains the resulting performance degradations will be severe even for feedback delays of only few ms and even for very moderate mobile speeds.
While these techniques help to reduce feedback the main issue of feedback delay is not solved this way. There more brute force methods are discussed to use fast fibres for inter Node B connections to avoid any additional delay in the processing chain.
Another option would be the definition of a specific fast feedback channel on physical layer similar to that for WCDMA power control.
Thus, there may be a need for a communication network element, a method for transmitting data, a communication network system, a program element, and a computer readable medium providing an improved performance of the communication network, in particular in a cooperative antenna system.